Effect of eukaryotic initiation factor 4F on AUG selection in a bicistronic mRNA

Artificial bicistronic mRNAs based on rabbit beta-globin and bacterial chloramphenicol acetyltransferase protein-coding sequences were tested for translation activity in a mouse astrocytoma cell-free extract. This cell extract exhibited an apparent preference for 5'-distal or internal initiation over 5'-proximal ("first AUG") initiation. 5'-Distal initiation appeared to be 5'-cap independent, suggesting that nonstandard initiation was responsible. This conclusion was based on a lack of inhibition of internal initiation by added cap analog and insensitivity of internal initiation to the presence or absence of a 5'-cap structure. Exogenous reticulocyte initiation factors were tested for effect on 5'-proximal initiation. The only factor with a significant effect was found to be eukaryotic initiation factor 4F, or the cap-binding protein. Addition of this factor promoted 5'-end initiation as evident by a general increase in 5'-proximal open reading frame (ORF) product relative to 5'-distal ORF product. The relative expression of 5'-proximal to 5'-distal ORFs in bicistronic or multicistronic mRNAs may very well be dependent on activity levels of eukaryotic initiation factor 4F and possibly other mRNA-dependent initiation factors.